The present invention relates generally to non-contact gauging systems. More particularly, the invention relates to an apparatus system and method for calibrating non-contact gauging systems.
Demand for higher quality has pressed manufacturers of mass produced articles, such as automotive vehicles, to employ automated manufacturing techniques that were unheard of when assembly line manufacturing was first conceived. Today, robotic equipment is used to assemble, weld, finish, gauge and test manufactured articles with a much higher degree of quality and precision than has been heretofore possible. Computer-aided manufacturing techniques allow designers to graphically conceptualize and design a new product on a computer workstation and the automated manufacturing process ensures that the design is faithfully carried out precisely according to specification. Machine vision is a key part of today's manufacturing environment. Machine vision systems are used with robotics aned computer-aided design systems to ensure high quality is achieved at the lowest practical cost.
Achieving high quality manufactured parts requires highly accurate, tightly calibrated gauging systems. Not only must the gauging system have a suitable resolution to discern a manufactured feature of interest, it must be accurately calibrated to a known frame of reference so that the feature of interest may be related to other features on the workpiece. Without accurate calibration, even the most sensitive, high resolution gauging system will fail to produce high quality results.
Keeping the gauging system properly calibrated is more easily said than done. In a typical manufacturing environment gauging systems and their associated robotic mounting structures may get bumped or jarred, throwing the system out of alignment. Also, from time to time, a sensor within the system may need to be replaced, almost certainly requiring reorienting and recalibrating.
One problem with gauging system alignment and calibration is the time required. Invariably, the entire manufacturing assembly line for a given part must be shut down and the workstation cleared whenever it is necessary to recalibrate the gauging system. In some instances this entails placing an independently measured (and very expensive) full-scale model of the workpiece in the workstation. This independently measured workpiece is sometimes called a master part. The master part is placed in careful registration with the external coordinate system of the workstation and then the gauging system sensor is trained on its assigned feature (such as a hole or edge). From the known position of the external coordinate system, the gauging system is recalibrated. Only then can the assembly line be brought back online.
Whereas the aforementioned calibration technique does work, there is considerable interest in a calibration technique that is quicker and easier to accomplish and that eliminates the need to rely on expensive master parts. To this end, the present invention provides a calibration system that can be used in a matter of minutes, instead of hours, and without the need for precisely manufactured master parts or theodolite equipment. A major advantage of the invention is that it allows the calibration of a system comprising a sensor mounted on the end of a robot arm to be checked or realigned between line shifts, without requiring the line to be shut down for an extended period. In addition to calibrating sensors, the calibration techniques contemplated by the present invention may also be used in the more general case to true-up or straighten the coordinate frame of a robotic system and to provide tool center point (TCP) calibration.
In another aspect the calibration system of the invention may be used to determine the appropriate calibration factors needed to compensate for link length changes and other mechanical changes in a robotic system. A robotic system typically employs several movable members, joined for pivotal or articulated movement. These movable members or links, being connected to one another, define geometric relationships by which the coordinate system of the robot gripper can be calibrated with respect to the coordinate frame of the robot base. These relationships depend, of course, upon the lengths of the links involved. Unfortunately, most materials change length as temperature changes. Many modem day robots are manufactured from aluminum, which has a substantial coefficient of expansion. Thus, a robotic system that is calibrated at a first temperature may not remain calibrated once the work environment drifts to a different temperature. Temperature fluctuations are quite prevalent in many manufacturing environments, hence loss of calibration due to link length change and other mechanical changes as heretofore been a frustrating fact of life.
The present invention provides a quick and convenient solution to the link length problem, using a plurality of the target structures described above. A robot equipped with a non-contact sensor is caused to move to different locations at which the target structures are disposed. By placing the target structures in known locations, the robot, with sensor in gripper, discovers each target in its field of measurement and is thereby calibrated in different states of arm extension. The system analyzes positional data obtained at each of the target stations and uses mathematical transformations to determine the current link lengths. This information may then be used to calibrate the system for the current ambient temperature. Because the system is quick and easy to use, it can be employed periodically to check and recalibrate the system without lengthy plant shutdown. This technique may also be used to compensate for changes of the sensor with temperature, as well as changes in rotary joints.
Briefly, a non-contact sensor is disposed on the movable member and emits structured light in a predefined planar configuration. A target structure preferably of a tetrahedron configuration is disposed within a field of view of the non-contact sensor. The target has a three-dimensional framework that defines at least three non-collinear, non-co-planar structural lines. The non-contact sensor for sensing the spacial location and orientation of the target structure also has an optical receiver for receiving reflected light emitted by the non-contact sensor. A coordinate translation system is connected to the non-contact sensor for calibrating the sensor to a target structure based upon the structured light reflected from the structural lines of the target structure.